A probe-holder armature of the kind mentioned above is known, for example from EP 0 882 896 A1, where a probe-holder armature is described with a sensor probe that is removably arranged in an immersion tube. The immersion tube is axially movable in a housing between a retracted rest position and a deployed working position in order to bring the anterior part of the sensor probe into contact with a measuring medium. The sensor probe in its installed condition provides a medium-tight closure of the immersion tube, for example by means of a ring seal that is arranged between the immersion tube and the sensor probe housing. If the immersion tube is deployed from the rest position into the working position when no sensor probe is installed, the absence of the medium-tight closure can have the undesirable consequence of allowing measuring medium to escape into the empty immersion tube and other parts of the probe-holder armature. The probe-holder armature according to the aforementioned reference is therefore equipped with a safety device which prevents axial movement of the immersion tube when no sensor probe is in place.
In the probe-holder armature according to EP 0 882 896 A1, an external screw thread in the rear portion of the sensor probe shaft serves to screw the sensor probe into a matching internal thread of the immersion tube. By turning the sensor probe into the immersion tube, a sleeve-like safety slide that is arranged coaxially inside the immersion tube is moved from a position where the immersion tube is locked against axial movement to a position where the immersion tube is free to move. As a consequence of this functional principle of the safety device, the immersion tube needs to have substantially the same length as the sensor probe. However, since the deployment depth of the immersion tube, i.e., the distance between the retracted rest position and the maximally deployed position of the immersion tube is of necessity shorter than the length of the immersion tube, the largest possible deployment depth is limited by the length of the sensor probe. Accordingly, with a probe-holder armature according to the aforementioned prior-art reference, a sensor probe of the very common length of 12 cm can be deployed to a maximum depth of slightly less than 12 cm. However, this is inadequate for many types of measurements, for example for measurements in large reaction vessels.
A probe-holder armature is described in EP 0 106 858 B1, where a sensor probe is arranged in an immersion tube that can be significantly longer than the sensor probe. In this case, the measuring tip of the sensor probe protrudes from the open front end of the immersion tube. The housing part that connects to a reaction vessel has at its front end a perforated cage into which the frontal part of the immersion tube—more specifically the sensor tip that protrudes from the immersion tube—is pushed in order to reach the deployed measuring position. A spring that is arranged in the cage applies a biasing force to the immersion tube towards the retracted rest position and thereby holds an anterior closure member of the immersion tube in a closed position, so that in order to deploy the immersion tube, it is necessary to first overcome the pre-tensioning force of the spring. While the deployment depth in this probe-holder armature is not limited by the sensor probe length, the deployment depth is nevertheless restricted by the displacement distance of the pre-tensioning spring, which in practice represents a considerable disadvantage. It also proves to be disadvantageous that the sensor probe or its holder serves as the means of opening the closure member when the sensor probe is deployed into the measuring position, in which case the front end of the sensor probe or its holder pushes the closure member open against the force of the pre-tensioning spring. Consequently, the front end of the sensor probe has to be particularly robust, or the holder has to be equipped with a pusher element at its front end. A pusher element of this kind represents one more component in the assembly and can furthermore interfere with the measuring function of the sensor probe. In addition, the spring in the cage is exposed to contamination by the measuring medium and difficult to clean. As a further aggravating disadvantage, the probe-holder armature according to EP 0 106 858 B1 has no protection whatsoever against being deployed when there is no sensor probe installed.